Takehisa Oikawa

A simulation model of the carbon cycle in land ecosystems (Sim-CYCLE): a description based on dry-matter production theory and plot-scale validation

Abstract In this paper, we present a new model of the terrestrial carbon cycle (Sim-CYCLE), with the objectives of retrieving the carbon dynamics of various terrestrial ecosystems and estimating their response to global environmental change. The model can be characterized in three ways. (1) It is a compartment model. Ecosystem carbon storage is divided into five compartments; foliage, stem, root, litter, and mineral soil. This approach made the model simple and sound, and allowed us to run the model on a broad scale; indeed, the simulation in this paper was performed using data available at the global scale. (2) It is a process-based model. Sim-CYCLE estimates net primary production (NPP) and net ecosystem production (NEP) by explicitly calculating such carbon fluxes as gross primary production (GPP), plant respiration, and soil decomposition on a monthly time-step; these fluxes are regulated by a multitude of environmental factors at the physiological scale. In relation to global change, responses to increased atmospheric CO2 and temperature should be modeled in a mechanistic manner. (3) It is a prognostic model. Sim-CYCLE is designed to be applicable not only to the simulation of an equilibrium state under given conditions, but also to the prediction of a transitional state under changing environmental conditions. Importantly, Sim-CYCLE is based on the dry-matter production theory, which enabled us to achieve the scaling-up from single-leaf to canopy and to conceptualize the growth process. Since the model includes both radiation and hydrological conditions, some indirect influences of the initial environmental change can be properly evaluated. We present a comprehensive model description and preliminary results confirmed at the plot scale: (1) intensively in four natural ecosystems and (2) extensively in global 21 sites. At each site, model parameters were calibrated to capture the observed carbon dynamics (e.g. productivity and carbon storage) at the equilibrium state. Successional growth patterns and seasonal variations in CO2 exchange were also examined in a qualitative manner. Sim-CYCLE successfully expressed the differences between tropical forest and boreal forest and between humid forest and arid grassland in terms of productivity and carbon storage. Next, we simulated transitional ecosystem carbon dynamics, in response to step-wise atmospheric CO2 doubling and disturbance regime. The simulated temporal patterns of carbon cycle were realistic and ensured that Sim-CYCLE is an effective tool for predicting the impact of global change.

Seasonal variations of the exchange of CO2 and H2O between a grassland and the atmosphere: An experimental study

Abstract Seasonal variations in the CO 2 and H 2 O fluxes were investigated experimentally over a grassland in central Japan during the growing periods of 1993 and 1994. The CO 2 flux over the grassland was measured by an aerodynamic method. The surface heat budget was routinely measured and the data were used to analyze the CO 2 and H 2 O exchange between the grassland and the atmosphere. Conditions were relatively cool and wet due to the prolonged rainy season from July to August 1993, being hot during the 1994 summer. The annual maximum value of sensible heat flux density was observed prior to the growing period on March 1993 and on April 1994, accounting for 40–50% of the net radiation. The annual maximum of latent heat flux density was observed in August, accounting for about 85% of the net radiation in both years. The latent heat flux observed in August 1994 was quite high, being 135 W m −2 (monthly mean, daily basis), while that in 1993 was 96 W m −2 . The grassland absorbed CO 2 during the period from the beginning of May to October in 1994, and maximum of daily net ecosystem CO 2 exchange during the growing period amounted to values more than 40 g CO 2 m −2 day −1 , in both 1993 and 1994. The dependence of daily net ecosystem CO 2 exchange on leaf area index (LAI) and incoming photosynthetic photon flux density Q p was investigated. When the Q p was higher than 45 mol m −2 day −1 , the daily CO 2 exchange strongly increased with an increase in LAI. The daily canopy surface conductance was calculated based on the Penman-Monteith equation, and it showed a linear increase with LAI when the LAI had small values (typically less than one). With an increase in LAI from one to four, the maximum level of surface conductance became dominated by water vapor pressure deficits.

Effect of carbon dioxide concentration on microbial respiration in soil

In order to assess the validity of conventional methods for measuring CO2 flux from soil, the relationship between soil microbial respiration and ambient CO2 concentration was studied using an open-flow infra-red gas analyser (IRGA) method. Andosol from an upland field in central Japan was used as a soil sample. Soil microbial respiration activity was depressed with the increase of CO2 concentration in ventilated air from 0 to 1000 ppmv. At 1000 ppmv, the respiration rate was less than half of that at 0 ppmv. Thus, it is likely that soil respiration rate is overestimated by the alkali absorption method, because CO2 concentration in the absorption chamber is much lower than the normal level. Metabolic responses to CO2 concentration were different among groups of soil microorganisms. The bacteria actinomycetes group cultivated on agar medium showed a more sensitive response to the CO2 concentration than the filamentous fungi group.

Effects of high light and low temperature during harsh winter on needle photodamage of Abies mariesii growing at the forest limit on Mt. Norikura in Central Japan

Abstract Coniferous evergreen firs Abies mariesii (A. mariesii) growing at the forest limit (near 2500 m altitude) on Mt. Norikura (36°61′N, 137°33′E, 3026 m altitude) in Central Japan are exposed to severe winter-stresses. Under such conditions, only the abaxial side of A. mariesii needles changes from green to reddish–brown in early spring, resulting in the death of the needles. Since this needle damage is only observed in shoots that protrude from the snow surface and not in those buried in snow or located at lower elevations, this phenomenon seems to be caused by the interaction of strong sunlight reflected from the snow surface and low temperature. We found that the damaged needles have increased in the de-epoxidation state of xanthophyll cycle because they contained large amounts of zeaxanthin, which appears when the leaves encounter a strong light stress, leading to the long-term down-regulation of PS II photochemistry. These results indicate that the needles acclimatize against the strong light during severe winter. Furthermore, ascorbate peroxidase (APX, E.C. 1.11.1.11) activity was found to decrease remarkably during critical subzero periods, while superoxide dismutase (SOD, E.C. 1.15.1.1) activity remains constant throughout the year. Based on these results, we discuss herein how A. mariesii growing at the forest limit of temperate zone responds to high light and low temperature in harsh winter conditions.

Zonal plant distribution and edaphic and micrometeorological conditions on a coastal sand dune

Some edaphic and meteorological conditions were examined to detect environmental gradients from shoreline to inland at the Kado-ori coast, Ibaraki, Japan, in 1989. Zonal distribution patterns of coastal dune plant species, including three ubiquitous perennials,Calystegia soldanella, Carex kobomugi andIschaemum anthephoroides, were described in relation to the environmental gradients. Environmental gradients were found in water availability, evaporative demand and soil-water salinity. Water availability, evaluated by thickness of capillary water layer, increased from 10 cm at 20 m to 48 cm at 85 m from the shoreline, reflecting the percentage of fine sand. Evaporative demand, which was evaluated by the evaporation rate from a wet black filter paper, decreased with increasing distance from the shoreline. Soil-water salinity was lowest (15 mmol/L) at 85 m from the shoreline and highest (90 mmol/L) at 30m. On the coast,C. soldanella, a salt-tolerant perennial, was distributed mainly in the environmentally harsh area 40–60 m from the shoreline.Ischaemum anthephoroides andC. kobomugi, less salt-tolerant perennials, occurred mainly 70–80 m from the shoreline, where environmental conditions were more hospitable.

Simulation of forest carbon dynamics based on a dry-matter production model

A microcomputer model for forest carbon dynamics with five functional comparments (atmosphere, foliage, woody-parts, roots and dead biomass in the soil) is constructed which incorporates dry-matter production processes of trees such as photosynthesis, respiration and allocation of photosynthate. The effect of photosynthesis rate at saturated light and dark respiration rate of a single leaf upon surplus production (Ps) is three-dimensionally illustrated as a function of cumulative leaf area index (LAI) and extinction coefficient of light. Probable values of the physiological parameters in this model are determined by repeated simulation experiments. The successional pattern during a period of 100 years is simulated, demonstrating stable and perpetual occurrence of a tropical rainforest ecosystem composed of three strata. The model is also analyzed in terms of response of relative initial density of trees, thereby displaying the law of constant final yield in a forest ecosystem. The model outputs of carbon fluxes and phytomasses at the steady state agree quite well with field data already obtained from a tropical rainforest at Pasoh.

The scheme for globalization of a process-based model explaining gradations in terrestrial NPP and its application

It is more or less obvious that a process-based net primary production (NPP) model tested for a stand can be applied over a global grid, but doing so, we face the problem—how to calibrate it at this scale. In this paper we demonstrate a scheme for such calibration with a process-based NPP model. First, we suggest that model parameters should be derived from a freely accessible database of NPP measurements. Then, we reduce the number of undefined model parameters to a single lumped parameter (SLP) and find its value for each database entry. Next, considering SLP as some characteristic of vegetation, we formulate an empirical model linking SLP and climate. And finally, we globalize the process-based model in hand by assigning SLP value to the nodes of geographical grid proceeding from the global fields of climatic variables.

Examination of the method for measuring soil respiration in cultivated land: Effect of carbon dioxide concentration on soil respiration

An acceleration of soil respiration with decreasing CO2 concentration was suggested in the field measurements. The result supporrs that obtained in laboratory experiments in our previous study. The CO2 concentrations in a chamber of the alkali absorption method (the AA-method) were about 150–250 parts/106 lower than that in the atmosphere (about 350 parts/106), while those observed in the open-flow IRGA method (the OF-method) were nearly equal to the soil surface CO2 levels. The AA-method at such low CO2 levels in the chamber appears to overestimate the soil respiration. Our results showed that the rates obtained by the AA-method were about twice as large as those by the OF-method in field and laboratory measurements. This finding has important consequences with respect to the validity of the existing data obtained by the AA-method and the estimation of changes in the terrestrial carbon flow with elevated CO2

Towards a model for projecting Net Ecosystem Production of the world forests

Abstract The global forest carbon budget constructed by Dixon et al. [Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., Wisniewski, J., 1994. Carbon pools and flux of global forest ecosystems. Science, 263, 185–190] from national carbon budgets suggests that forests in mid- and high-latitude regions simultaneously entered into the phase of recovery and formed a significant sink for atmospheric carbon in the 1980s. However, constructing a carbon budget from forestry statistics gives rise to the question: what is the true value of the factor which converts the wood volume into the carbon stock? One may avoid this question by merging global carbon cycle modelling and forest inventory. This approach is illustrated by the case study of Japanese forests and it concludes that the key to projecting Net Ecosystem Production (NEP) of world forests lies with their age structure.

Estimating terrestrial NPP: what the data say and how they may be interpreted?

Abstract The differences in the estimates of terrestrial Net Primary Production (NPP) clearly manifest the demand for a standardized data set and for a standardized way of its treatment. Starting from 1970, the estimates of terrestrial NPP range from 40 to 80 Gt C/year, with the central value of 60 Gt C/year. The latter value, which is commonly used in studies of the global carbon cycle, provides the basis for many conclusions. However, authors who calculated these estimates made no statements about reliability, and thus there is no certainty in 60 Gt C/year as well as in other estimates of the terrestrial NPP. Analyzing the database on NPP that was originally synthesized by a group of scientists from the University of Osnabruck (Germany) led by H. Lieth in the 1970s and 1980s, we conclude with 90% confidence that terrestrial NPP falls within 50–70 Gt C/year in the context of Miami model. This can hardly be the ultimate answer, as different NPP models give us different contexts for calculating terrestrial NPP. In view of the fact that global scale estimates are essentially contextual, we suggest that the uncertainty of terrestrial NPP arises from NPP models more likely than from NPP data.